For integral in-mold production of labeled, resin-molded containers, heretofore employed is a process of previously inserting a blank or a label into a mold, and then molding a container in the mold in a mode of injection molding, blow molding, differential pressure molding or foam molding to thereby label the container (e.g., JP-A-58-69015). As such in-mold labels, known are gravure-printed resin film, offset multicolor-printed synthetic paper (e.g., JP-B-2-7814, JP-A-2-84319), or aluminium label produced by lining aluminium foil with polyethylene or ethylene-vinyl acetate copolymer on its back and then gravure-printing the foil on its surface.
In-mold labels are printed with product name, manufacturer name, dealer name, character, bar code, instructions for use, etc., and then used for in-mold production. For printing them, employable are various printing methods of sheet offset printing, rotary offset printing, gravure printing, flexographic printing, letterpress printing, screen printing; but from the sharpness of the printed matters and the production costs, sheet offset printing is much used.
However, in a method for producing label-modified, resin-molded articles according to an in-mold production process of using the above in-mold label, when the antistatic performance of the label is insufficient, then the method is defective in that it causes an electrostatic trouble in label production in a low-humidity environment in winter. In a label-printing process where in-mold labels are printed in a mode of sheet offset printing, the sheets being processed into in-mold labels may have a printing trouble owing to the generation of static charges.
A sheet offset printing system is composed of three parts of a paper feeding part, a printing part and a paper delivery part. In the paper feeding part, the sheets to be printed are fed to the printing part one by one, in which each sheet is printed with ink, and then the printed sheets are conveyed to the paper delivery part, and are thereafter stacked up. In the printing part, a predetermined amount of ink is metered and fed from an ink supply to each sheet in accordance with a pattern, then applied to the printing area of a printing plate with a pattern, and transferred onto a rubber printing blanket for sheet offset printing. FIG. 1 shows a mechanism of ink transferring onto a sheet, in which a sheet (1) is sandwiched between a rubber blanket (2) for sheet offset printing and a metal impression drum (3) that synchronizes with it, and ink is thus transferred onto the surface of the substrate layer of the sheet (1). When the sheet (1) is peeled from the blanket (2) for sheet offset printing, the surface of the sheet may have static charges generated therearound. As a result, when the antistatic capability of the sheets for in-mold labels is poorer, then the sheets could not be stacked up regularly owing to the electrostatic repulsion thereof. When static charges are accumulated in the paper delivery part, it may take a lot of time to stack up them owing to their repulsion and, as a result, the printing speed could not be increased and the process is therefore inefficient.
When labels are fed into a mold with an automatic label feeder, then the static charges between the stacked labels could not be removed, therefore causing some problem in that two or more labels may be fed at the same time into a mold to give irregularly-labeled, resin-molded articles (rejected products) or the labels would drop and could not be used efficiently.
To solve these problems, used are in-mold labels in which a kneadable low-molecular antistatic agent is kneaded in the heat-sealable ethylenic resin layer, or those in which a low-molecular antistatic agent is applied onto the surface of the heat-sealable ethylenic resin layer and dried to form an antistatic layer thereon.
However, the in-mold labels of both types have a drawback in that the antistatic performance thereof could not last long and its durability is poor. In addition, the former in-mold labels have another problem in that the antistatic agent component therein may migrate or concentrate in the surface of the heat-sealable resin layer, therefore greatly interfere with the fusibility of the heat-sealable resin to containers with the result that the labels could not fuse to containers at all or the labels having fused to containers may blister.
To solve the above problems, proposed is a method of adding a polyether-ester-amide having a long-lasting non-adhesive antistatic capability, to a heat-sealable resin (e.g., JP-A-11-352888).
However, the method has a problem in that, in a process of kneading the additive in a heat-sealable resin and extruding it through a T-die of an extruder to produce labels, it may deposit and degrade around the outlet port of the T-die to give a large amount of burr, or it may deposit on and soil the roll surface in the production line that is in contact with the heat-sealable resin layer with the result that the burr and the dirty deposit may drop off to cause defects of the produced films, and, as a result, the production line must be frequently stopped to clean the die tip and the roll surface.